You could try normal insurance companies, and if those can't help, then an insurance specialist like Adrian Flux should be able to help. I suspect the insurance will cost more than a normal FL1, and unless it has all its installation and gas test certificates, it might be uninsurable.
I'm assuming it's LPG converted? It might even be worth taking the LPG kit out, especially as LPG is now being phased out as a road fuel.